[TowerTalk] Re:N4KG 40-2CD mods
JACK
w1wef@capecod.net
Tue, 26 Sep 2000 08:09:42 -0400
I have done Tom's mods to beef up both of my 40-2CD's and they have stood up FB.
I think those simple steps are well worth doing. JACK
W1WEF------------------------------------------
>
> Date: Mon, 25 Sep 2000 18:29:53 -0600
> From: n4kg@juno.com
> Subject: Re: [TowerTalk] Your opinion on 40 mtr antenna
>
> The CC 402CD can be beefed up by double walling
> the inner pieces of the elements or you can buy
> the NEW heavy duty version. My beefed up
> 402CD survived estimated 80+ mph winds.
> GREAT antenna.
>
> If you would be satisfied with a rotary dipole,
> Gamma or Omega matching the boom of the TH6
> works VERY WELL. See below.
>
> de Tom N4KG
>
> BOOM MATCHING as a ROTARY DIPOLE on 40 / 30 Meters - N4KG
>
> The loaded boom dipole trick works very well indeed! A 24 ft boom end
> loaded by 20M (full size or trapped) elements is self resonant very near
> 40M ( resonance is not needed for efficient radiation). I have matched
> my TH6 boom first for 40M and now use it on 30M where it has accounted
> for 290 countries. Shorter booms will also work with slightly less
> bandwidth.
>
> The radiation resistance of a dipole depends on it's height above ground
> so using someone else's exact matching system may not give identical
> results unless the antennas are at the same height also.
>
> A better approach is to describe the TECHNIQUE and make your own
> matching system, tuned in place on the tower.
>
> First, a matching ARM needs to be attached to the boom and brought
> back to the mast. I like to use 3/4 inch CATV hardline as a semi-rigid
> piece of tubing that can be easily bent and formed. (Besides, it's
> free!)
>
> I use a sloping feed, from near the 15M director back to the mast about
> 12 to 15 inches above the boom, and insulated from the mast by a piece
> of slotted PVC over the aluminum tubing. I use 1/8 inch nylon rope to
> lash the tubing/PVC/mast together. Smash the end of the tubing and
> use a hose clamp to attach the arm to the boom. The arm may also be
> suspended below the boom if the antenna has enough clearance
> above the top of the tower. Either a sloping or parallel arm will work.
>
> I like using an OMEGA match (see ARRL Antenna Book) because it can
> easily match lower impedances up to 50 Ohms AND tune out the inductive
> reactance of the arm. With this system, it is NOT necessary to find the
> 50
> Ohm tap point (which can be a real pain!). If the beam is already on the
>
> tower, place the arm attachment point out as far as you can safely reach.
>
> The OMEGA match uses two capacitors, one from the arm to "ground"
> (the center of the boom or mast, assuming a good boom to mast connection)
> and another from the arm to the center of your feedline. The braid of
> the
> feedline is connected to the "ground"/mast/boom-center.
>
> To determine the necessary capacitor values, I mounted two 300 pf
> variable capacitors on a piece of plexiglas, with a plexiglas front, and
> pointer knobs. A piece of 3" by 5" card (paper) is mounted behind the
> knobs and lines drawn with calibration marks. (It helps to have a
> capacitance or impedance meter available for calibration).
>
> To match the arm, I mounted two receiving-type 300 pF variable
> capacitors
> on a piece of plexiglas and connected the stators together. A short wire
> runs from this junction to the end of the arm where a hose clamp can be
> used to make the connection. The rotor of one capacitor goes to the
> "ground" connection at the mast or center of the boom. The rotor of the
> other capacitor goes to the center of your feedline. The braid of your
> feedline goes to the "ground" at the center of the boom or mast. Keep
> these leads as short as possible.
>
> The BEST way to tune the capacitors is with a battery powered
> Antenna Analyzer. Your transmitter (at reduced power) with a helper,
> two meter radio, and SWR meter (at the antenna) will also work.
> It is a good idea to wear heavy rubber gloves and have good
> communication (KEY, DONT KEY, etc.) with your helper.
>
> Once the capacitor values are known, I make weather proof capacitors
> from solid dielectric coaxial cables such as RG-8, RG-213, or even RG-59.
>
> (Solid dielectric cables have much higher breakdown voltage ratings than
> foam). Fifty ohm cables are approximately 30 pF per foot while 75 ohm
> cables are around 21 pF per foot. To prevent arcing at the far end of
> the
> cable, I trim off 1/2 inch of braid and tape the end. The cable can be
> coiled. I like to tape the coiled cables to the matching arm (isolated
> from
> the boom and mast). Be sure to attach the BRAID of the cable capacitors
> to the matching arm. (This prevents arcing from the braid through the
> outer jacket which has only a 600 V rating). The center conductor of the
>
> shunt capacitor (cable) is then connected to the mast or boom center.
> The
> center conductor of the series capacitor (cable) is connected to the
> center conductor of your feedline. Again, the braid of the feedline is
> connected to the center of the boom (or to the mast). It helps to draw
> a picture!
>
> There are several advantages to matching the boom of a Yagi as a
> rotary dipole compared to using an inverted Vee suspended below
> the Yagi. One obvious advantage is that the antenna can be rotated to
> maximize radiation in the desired direction. A less obvious fact is that
>
> a flat horizontal dipole can have up to 3 dB more gain than an inverted
> vee because radiation off the ends is minimized and interactions with
> other antennas is minimized.
>
> This technique of matching a Yagi boom as a rotary dipole has been
> successfully copied by several others with good success. My own
> systems continue to perform well after 20 years. Using various
> rotary dipoles at 80 ft, I have worked over 300 countries on 40 meters.
> Optimum heights range from 80 to 90 feet, where the radiation resistance
> of a 40M dipole goes through a minimum, thus maximizing the current
> (and GAIN) of the dipole. Because of this impedance / current / gain
> relationship to height, further increases in height actually yield LESS
> gain until approaching a height close to 1 wavelength (140 ft.) !
>
> If you have followed me this far, have an adventurous spirit, and are
> looking for a good 30 meter antenna in addition to 40M, I see no
> reason that one could not put a second matching arm to the opposite
> side of the mast and tuning that arm to match on 30M. If separate
> feedlines are used, be sure to NEVER connect both feedlines to
> separate radios at the same time!
>
> Try it, you'll like it !
>
> GL, Tom N4KG 10 / 23 / 97
> ...............................
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> ------------------------------
>
> End of Tower Talk Digest V3 #385
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